docs: def sky_bbox_ul(self):
          """
          The sky coordinates of the upper-left vertex of the minimal
          bounding box of the source segment, returned as a
          `~astropy.coordinates.SkyCoord` object.
  
          The bounding box encloses all of the source segment pixels in
          their entirety, thus the vertices are at the pixel *corners*.
          """
  
          if self._wcs is not None:
              return pixel_to_skycoord(self.xmin.value - 0.5,
                                       self.ymax.value + 0.5,
                                       self._wcs, origin=0)
          else:
              return None